Activated sludge sewage treatment



plri 23, 1935. W, RAlsCH ACTIVATED SLUDGE sEwAGE TREATMENT Filed Aug. 30, 1932 Patented pr. I2?, 1935 rATsNT OFFICE ACTIVATED SLUDGE SEWAGE TREATMENT William Raisch, Forest Hills, N. Y., assignor to Municipal Sanitary Service Corporation, a corporation of New York Application August so, 1932, sen'ai No. 631,053.

9 Claims.

This invention relates tosewage treatment by activated sludge processes.

In such processes of sewage treatment, the,

degree of puriiication required depends largely upon the volume of diluting water in the stream or body of water into which the purified eluent flows. Furthermore, regardless of the amount of diluting water available for disposal of the treat'- ed sewage, it is necessary to adopt a process which will be odorless and which will require a limited area for the necessary plant. The trend is to make processes-more reliable by mechanical means to get away from uncertain biological actions. In substantially all cases the raw sewage is rst subjected to screening and then to sedimentation. The screening step removes bulk solids and a portion of the remaining solids suspended in the sewage is removed by sedimentation in the second step.

Where bacteriological treatment is contemplated as in the activated sludge process, it is essentialtov remove as much of the coarse solids as possible before the bacteriological oxidizing treatment 1is started. According to the process heretoforejn extensive use, the sewage after treatment by screening and by sedimentation (usually about 1 to2 hours) in a primary settling tank is conveyed to aeration tanks in which it is subjected to the action of aerobic bacteria encouraged-by .aeration as by forcing air from a suitable air compressor into the aeration tanks in such a manner as to mix, thoroughly with the sewage therein. After aeration andl oxidation for a considerable length of time (for example, six to eight hours), the sewage is transferred to a secondary settling tank (where it may remain 'aother period of 11/2 to 2 hours) from which thev overow of treated sewage or the final aiuent is discharged, vfor example, into a stream or body oi water. i

From the primary settling tank or sedimentation unit, the raw sludge in thebottom thereof is transferred, by pumping, if required, to a suitable .place for disposal, such as sludge drying beds. Theactivated sludge is drawn from the bottom of the secondary settling tank and the greater proportion thereof is also transferred to -a suitable point ofdisposal such as the sludge drying beds to which the sludge from the primary settling tank is also transferred. In order to keep up the proper action and cultivate the liquid in the aeration tanks, part of the activated sludge 'taken from the secondary settling tank is' returned into the aeration tanks. Usually"A about 25% ciLthe activated sludge from the secondaryv settling tank is returned to the aeration tanks. Inasmuch as the amount of air required in the activated sludge process depends almost entirely upon the amount of organic solids in the sewage, which solids are to be oxidized, the amount of air necessary for use in connection with processes heretofore in use is very great.V I

While it is true/that a considerable quantity of such organic solids are removed from the primary s'ettling tank or Atanks before the sewage is passed through the aeration tanks, there is a vlimit to the'amount of solids that can be re- As a matter of fact, it is considered that about 50% of the suspended moved in this manner.

solids contained in the original raw sewage can be taken out by the screens and in the primary settling tank or tanks.

If a greater proportion of removal is desired, other means must be employed such, for example, as chemical precipitation which, however, gives rise to very seriousl disadvantages inasmuch as the chemicalsv used in chemical precipitation interfere materially with the bacteriological activities upon which the activated sludge process is based. Another method might be to increase the period of sedimentation but in doing so the area covered by the tanks is proportionately increased making the cost prohibitive and on the whole impracticable.

It is an important object of the invention to provide improved methods of activated sludge treatment of sewage -Another important object of the invention relates to the `provision of improved apparatus for carrying out activated sludge processes. Further objects relate to the provision of apparatus and'process which are very flexible in operation. It is a feature of. the invention to provide a process whereby the cost of operation will be much lower than for processes heretoforeusedi Other features relate to reductions in cost of the plant and in cost of operation. Also, various features of the invention relate to provisions whereby superior effects may be obtained at a minimum of cost.

invention may be carried out in the following manner.

"remove a much larger quantity of sludge than usual from the sewage before the sewage .passes into the aeration tanks, resort may be made to forced filtration as by means of a vacuum filter or vacuum iilters, to which is fed as large a proportion as is found desirable of the overflow from the primary settling tank or tanks, the remainder of such overflow' being passed directly to the aeration tanks. The vacuum iilter or filters reand a pipe move substantially all of the suspended solids from the sewage fed thereto and the filtrate, substantially free from suspended solids, passes from the vacuum lter or filters to the aeration tank. The solid matter removed by the vacuum filter may be disposed of in any suitable manner. In this way the amount of suspended solids in the sewage entering the aeration tanks is'very much reduced, thus reducing the requirements for compressed air and rendering the operation very much cheaper inasmuch as the power required for a vacuum filter is very much less than that which would be required for additional air-compressor capacity.

If desired, practically of the solids in all the sewage could be removed by the vacuum filters. If, however, it be undesirable to remove all of the suspended solids before passing the liquid to the aeration tanks, then, as already referred to, a part of the raw sewage may be by-passeddirect to the aeration tanks, the balance going through the lters and then passing to the aeration tanks. It will be evident that the process of the present invention is very nexible and can be 'adjusted to meet any condition of septicity. This flexibility is a very important feature in any sewage process, inasmuch as raw sewage has characteristics far different from sewage which has been aerated for six or eight hours.

vOther objects, features and advantages will appear upon consideration of the following description and of the drawing, in which the figure is a diagrammatic representation of a preferred form of apparatus for carrying out the method.

As illustrated in the drawing, raw sewage is fed through a pipe or duct I0 to a bar screen II which serves to remove the bulky solids which would otherwise cause trouble in the later processes, in that they are often of entirely different character from the other solids in the sewage. From the bar screen II,- the sewage passes through a pipe or duct I2 to a primary settling tank I3.

In settling tanks of the character indicated the sewage is brought in beneath the level of the liquid so as to not disturb the surface thereof and the solids gradually settle toward the bottom and are gradually Worked toward the center by Scrapers or the like on the arms of a rake structure which is rotated slowly above a vertical axis. The sludge collected at the bottom of the primary settling tank is drawn out of the bottom thereof and passes through a duct or pipe I4 to sludge drying bed I5. When no sludge is being passed the pipe I4 may be closed by valve I6.

The settling action in the tank I3 serves to clarify the liquid in the upper part thereof and, as solids tend to drop, the clarified liquid will overow and be discharged through a pipe or duct I1. The pipe I1 is provided with a valve I8 and is connected with an aeration section I9 which may include a plurality of tanks 20 of the spiral flow or channel type. Heretofore it has been a general practice to pass the sewage directly from the primary settling tank to aeration tanks. This, however, requires the bacteriological treatment in the aeration tanks of a relatively large and constantly varying quantity of solids thus requiring the supply to the aeration tanks of a large quantity of compressed air which may be supplied thereto by means of a compressor 2|, a compressed air tank or receiver 22 23 controlled by valve 24.

It will be evident that, if a large proportion of the suspended solids'f say '75% or possibly 100%, be removed in advance, the solid matter to be oxidized in the aeration tanks will be very much less in quantity and consequently very much less air will be required, thus cutting down very substantially the cost of operation. The present invention involves the removal Vof such suspended solids and to this end provision may be made of one or more vacuum lters 25 receiving settled sewage from the line I1 back of the valve I8 and through a pipe 26 controlled by valve 21. These vacuum filters are mechanical units and permit a removal of substantially 100% of the settled solids. Preferably these vacuum filters are of the Feinc type, which type is based on the patent to Wright & Young, Number 1,472,574, granted October 30, 1923, the residue being discharged into one or more troughs or receptacles 28 and the filtrate being discharged through a pipe 29 controlled by a v alve 3D into the pipe I 1 near the aeration tanks. ing more or less of the settled sewage through the vacuum filters the amount of solids to be oxidized in aeration tanks may be controlled substantially as desired and the control may bevaried at a moments notice. by this method I can prevent aeration tanks.

After treatment in the aeiation tanks which It will be seen that overloading of the may have lasted for several hours, the sewage 1 containing activated sludge is discharged through a pipe 3| into a secondary settling tank 32 which may be of the same-type as the primary settling tank, the sludge being discharged from its bottom through a pipe 33 and the clarified eiiluent being discharged through a pipe 34 into any suitable means for disposal such as a stream. The greater part of the activated sludge may be passed through the pipe 33 to the sludge drying beds.

It is desirable however to return a portion of the activated sludge, which now contains aerobic bacteria, to the aeration tanks to assist in the bacteriologicaly action in these tanks. .To this end the pipe 33 is provided with a branch 40 and the proportion of the activated sludge to be returned to the aeration tanks may be controlled by valve 35 in the line 33 between the branch 4U and the sludge dryingbeds and a valve 36 in the branch 40. Sludge entering the branch 40 may be returned to the aeration tanks in any suitable manner, as by means of a pump 31 connected with a branch 40 to receive the activated sludge therefrom and with a pipe or duct 38 which discharges into the aeration tanks through a portion of the pipe I1 with which it connects. The pipe 38 may also be provided with a valve 39.

It will be evident that, by the introductionof one or more vacuum filters 25, it will be made possible to use a much smaller air compressor installation. This-will reduce very substantially the cost of installation in view of the relatively small cost of the vacuum filters as compared with the saving in view of the smaller required size of the air compressor installation or by reducing y It will be evident that by pass- Leoaose without departing from the true spirit and scope of the invention.

Having thus described my invention, I claim 1. An activated sludge process of sewage treatment, comprising removal of solids from the sewage by sedimentation, forced filtration of part of the resulting eluent, and mixing of the resulting filtrate and the rest of said effluent, and aeration of the mixture thus formed.

2. An activated sludge process of sewage treatment, comprising removing solids by settling, suction-filtering part of the resulting eluent, bringing together the filtrate and the remainder of said eflluent and subjecting them to aerobic bacterial action. l

3. An activated sludge process of sewage treatment, comprising removing the larger solids, forced filtering substantially half of the resulting eiuent, bringing together the resulting ltrate and the remainder of said eiiluent and subjecting them to aerobic bacterial action.

4. An activated sludge process of sewage treatment, comprising removing the larger solids by settling, suction-filtering substantially half of the resulting eluent, bringing together the resulting filtrate and the remainder of the eiliuent, and subjecting themto aerobic bacterial action.

5. In apparatus for continuous activated sludge sewage treatment, a settling tank, means for drawing 01T sludge from the bottom of said tank, aeration means; means including a duct for continuously supplying supernatant liquid from the settling tank to the aeration means, a filter in said duct, a by-pass around said lter, and means to control the relative amounts of sewage passing through the by-pass and the lter.

6. An activated sludge process of sewage treatment, comprising removing the larger solids, further reducing the proportion of solidsqin the resulting eliiuent by forced lt'ration of part of said eiuent and bringing together the resulting filtrate and the remainder of said efuent thereby producing a modified eiuent, and subjecting the modied eluent to aerobic bacterial action.

7. In apparatus for continuous activated sludge sewage treatment, settling means, means for drawing oft' sludge from said settling means, aeration means, means -for passing supernatant liquid from said settling means to said aeration means, filtration means interposed in such liquidpassing means, means for by-passing supernatant liquid around said filtration means, and means for controlling the relative amounts of supernatant liquid passing through the bypassing means and the filtration means.

8. An activated sludge process of sewage treatment comprising the steps of removing the larger solids by sedimentation in a sedimentation tank, thereafter aerating the sewage eluent from said tank in aeration apparatus, and subsequent to `such sedimentation but prior to such aeration -removing by forced filtration variable additional amounts of solids from said eflluent to maintain the solid content of the sewage efuent delivered to the aeration apparatus within the maximum capacity of the aeration apparatus.

9. An activated sludge process of sewage treatment comprising removing the larger solids in apparatus adapted to that purpose, subsequent-v ly aerating in aeration apparatus the sewage efluent from the larger-solid-removing means, and, subsequent to the larger-solid-removing operation and prior to the aeration, removing by forced filtration variable amounts of additional solids from said sewage effluent to maintain the solid content of the sewage eluent delivered to the aeration apparatus within the maximum capacity thereof. I

. WILLIAM RAISCH. 

